Clock Recovery and Equalizer Estimation In a Multi-Channel Receiver

ABSTRACT

A method of performing clock recovery and equalizer coefficient estimation in a multi-channel receiver may include recovering, at a first clock recovery unit, a first clock signal associated with a first channel. The method may include estimating a first set of coefficients for a first equalizer associated with the first channel using the first clock signal. The method may include passing the first clock signal to a second clock recovery unit associated with a second channel. The method may also include recovering, at the second clock recovery unit, a second clock signal associated with the second channel using the first clock signal as a reference clock signal. The method may also include passing the first set of coefficients as initialization coefficients to a second equalizer associated with the second channel. The method may also include estimating a second set of coefficients for the second equalizer using the initialization coefficients.

FIELD

Some embodiments described herein generally relate to clock recovery andequalization implementations in multi-channel receivers.

BACKGROUND

Unless otherwise indicated herein, the materials described herein arenot prior art to the claims in the present application and are notadmitted to be prior art by inclusion in this section.

Signals transmitted through different channels on a multi-mode fiber(MMF) link may experience inter-symbol interference (ISI). Some channelsmay have severe ISI, causing eye openings in associated eye diagrams tobe closed. It may be difficult for a receiver to detect signalstransmitted through the channels with severe ISI.

The subject matter claimed herein is not limited to embodiments thatsolve any disadvantages or that operate only in environments such asthose described above. Rather, this background is only provided toillustrate one exemplary technology area where some embodimentsdescribed herein may be practiced.

BRIEF SUMMARY OF SOME EXAMPLE EMBODIMENTS

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential characteristics of the claimed subject matter, nor is itintended to be used as an aid in determining the scope of the claimedsubject matter.

Some example embodiments described herein generally relate to clockrecovery and equalization implementations in multi-channel receivers.

In an example embodiment, a method of performing clock recovery andequalizer coefficient estimation in a multi-channel receiver isdescribed. The method may include recovering, at a first clock recoveryunit, a first clock signal associated with a first channel. The methodmay also include estimating a first set of coefficients for a firstequalizer associated with the first channel using the first clocksignal. The method may also include passing the first clock signal fromthe first clock recovery unit to a second clock recovery unit associatedwith a second channel. The method may also include recovering, at thesecond clock recovery unit, a second clock signal associated with thesecond channel using the first clock signal as a reference clock signalto the second clock recovery unit. The method may also include passingthe first set of coefficients as initialization coefficients to a secondequalizer associated with the second channel. The method may alsoinclude estimating a second set of coefficients for the second equalizerusing the initialization coefficients.

In another example embodiment, a multi-channel receiver is described.The receiver may include a first clock recovery unit, a firstcoefficient estimation unit, a second clock recovery unit, and a secondcoefficient estimation unit. The first clock recovery unit may beconfigured to recover a first clock signal associated with a firstchannel. The first clock recovery unit may be configured to pass thefirst clock signal to a second clock recovery unit associated with asecond channel. The first coefficient estimation unit may be configuredto estimate a first set of coefficients for a first equalizer associatedwith the first channel using the first clock signal. The firstcoefficient estimation unit may be configured to pass the first set ofcoefficients to a second coefficient estimation unit. The second clockrecovery unit may be configured to recover a second clock signalassociated with the second channel using the first clock signal as areference clock signal to the second clock recovery unit. The secondcoefficient estimation unit may be configured to estimate a second setof coefficients for a second equalizer associated with the secondchannel using the first set of coefficients as initializationcoefficients for the second equalizer.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by the practice of the invention. Thefeatures and advantages of the invention may be realized and obtained bymeans of the instruments and combinations particularly pointed out inthe appended claims. These and other features of the present inventionwill become more fully apparent from the following description andappended claims, or may be learned by the practice of the invention asset forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of thepresent invention, a more particular description of the invention willbe rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope. The invention willbe described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1A is a block diagram of an example multi-channel transmitter;

FIG. 1B is a block diagram of an example multi-channel receiver;

FIG. 2 is a block diagram of an example process of performing clockrecovery and equalizer coefficient estimation in the multi-channelreceiver of FIG. 1B;

FIG. 3 includes a graphic representation of example bandwidth-wavelengthdependence on an optical link;

FIG. 4 includes graphic representations of example eye diagramsassociated with different wavelengths on an optical link;

FIG. 5 shows an example flow diagram of a method of performing clockrecovery and equalizer coefficient estimation in a multi-channelreceiver; and

FIG. 6 is a block diagram that illustrates an example computing devicethat is arranged for implementing digital signal processing techniquesin a data communication system.

DETAILED DESCRIPTION OF SOME EXAMPLE EMBODIMENTS

Embodiments described herein generally relate to clock recovery andequalization implementations in multi-channel receivers.

Some embodiments described herein may utilize dependence (orcorrelation) between neighbor channels on a multi-mode fiber (MMF) linkfor reliable clock recovery and equalizer convergence at a wavelengthdivision multiplexing (WDM) receiver. The MMF link may be optimized forperformance at a wavelength of a first channel (e.g., 850 nanometers(nm)). Accordingly, compared to other channels with other wavelengths(e.g., 880 nm, 910 nm, 940 nm), the MMF link may have a widest effectivebandwidth at the wavelength of the first channel (e.g., 850 nm).

Initially, a receiver of the MMF link may recover a first clock signalassociated with the first channel and may estimate first coefficientsfor a first equalizer associated with the first channel. Next, thereceiver may use the first clock signal as a reference clock signal torecover a second clock signal associated with a second channel (e.g.,880 nm). The receiver may also use the first coefficients to aid inestimating second coefficients for a second equalizer associated withthe second channel. The receiver may use the second clock signal as areference clock signal to recover a third clock signal associated with athird channel (e.g., 910 nm). The receiver may also use the secondcoefficients to aid in estimating third coefficients for a thirdequalizer associated with the third channel (e.g., 910 nm). Similarly,the receiver may perform similar operations to recover clock signals andto estimate coefficients for equalizers associated with any otherchannels of the MMF link. As a result, reliable clock recovery andequalizer convergence may be achieved for all WDM channels, includingthose with severe inter-symbol interference (ISI).

In some embodiments, the receiver may include: a demultiplexer (DEMUX)that may be configured to receive a laser beam from an optical link,where the optical link may be optimized for a particular wavelength andthe laser beam may include one or more optical carrier signalsassociated with other wavelengths; a first clock recovery unitconfigured to receive a first optical carrier signal of the opticalcarrier signals from the DEMUX and to recover a first clock signal fromthe first optical carrier signal; and a second clock recovery unitconfigured to receive a second optical carrier signal of the opticalcarrier signals from the DEMUX, to receive a reference clock signal fromthe first clock recover unit, and to recover a second clock signal usingthe reference clock signal. The first optical carrier signal may beassociated with a first wavelength. The second optical carrier signalmay be associated with a second wavelength. The first wavelength may bebetween the second wavelength and the particular wavelength. Thereference clock signal may be the first clock signal. The receiver mayfurther include a first coefficient estimation unit configured to:estimate a first set of coefficients for a first equalizer associatedwith the first channel using the first clock signal; and pass the firstset of coefficients as initialization coefficients to a second equalizerassociated with the second clock recovery unit.

Technologies described herein may be generalized for a receiver with anynumber of WDM channels. As a channel density in the receiver increases,similarity of fiber channel responses of neighbor channels may alsoincrease. Thus, performance of the technologies described herein may beimproved at the receiver.

The technologies described herein may be implemented in communicationmodules that may communicate data at a data rate of 40 Giga bits persecond (Gb/s), 100 Gb/s, and/or another suitable data rate in MMF WDMapplications. The technologies may provide a sub-system with improvedclock recovery and equalizer convergence, which may enable a longerreach of the optical link and/or a more reliable operation at thereceiver. The technologies described herein may also be applicable tosingle mode fiber (SMF) systems. For example, some data center opticalmodules may operate near a zero dispersion wavelength of about 1310 nmand may use WDM. Some of these WDM channels may experience a worst casechromatic dispersion, while some other channels near the zero dispersionwavelength may experience a lower dispersion.

The technologies described herein may include a system that may includea non-return-to-zero (NRZ) transmitter and a NRZ receiver, with ananalog feed-forward equalization decision feedback equalization(FFE-DFE) equalizer at the receiver. A more complex system with adigital-to-analog converter (DAC), an analog-to-digital converter (ADC),and various advanced modulation techniques may also be applied herein.

Reference will now be made to the drawings to describe various aspectsof some example embodiments of the invention. The drawings arediagrammatic and schematic representations of such example embodiments,and are not limiting of the present invention, nor are they necessarilydrawn to scale.

FIG. 1A is a block diagram of an example structure of a multi-channeltransmitter (transmitter) 100, arranged in accordance with at least someembodiments described herein. The transmitter 100 may be configured totransmit data (e.g., streams of data 112 a-112 d discussed herein) to amulti-channel receiver through an optical link 150. The optical link 150may include an MMF fiber such as OM3, OM4, or any other suitable fiber(e.g., an SMF fiber).

The transmitter 100 may include: a transmitter (TX) processing module103 that includes one or more TX processing units 102 a-102 d (alsoreferred to individually or collectively as TX processing unit 102 or TXprocessing units 102); a laser array 107 that includes one or more laserdiodes 108 a-108 d (also referred to individually or collectively aslaser diode 108 or laser diodes 108); a multiplexer (MUX) 110; one ormore other suitable transmitter components; or combinations thereof.

Although the transmitter 100 is illustrated as a 4-channel transmitterin FIG. 1A, the transmitter 100 may include more than 4 channels orfewer than 4 channels. In some embodiments, the transmitter 100 mayadditionally include receiving components that enable the transmitter100 to act as a transceiver for conducting bi-directional communicationsthrough one or more optical links (e.g., the optical link 150).

In some embodiments, one or more of the TX processing units 102 mayinclude a TX digital signal processing (DSP) unit. For example, the TXprocessing unit 102 may include a discrete multi-tone (DMT) TX DSP unitor an orthogonal frequency division multiplexing (OFDM) TX DSP unit.

In the depicted embodiment, each of the TX processing units 102 mayreceive a respective stream of data 112 a-112 d (e.g., a respectivestream of digital data bits), hereinafter “stream of data 112” or“streams of data 112,” and may process the respective stream of data 112to output a respective signal in a form suitable for driving a laserdiode. For example, the TX processing unit 102 a may receive a stream ofdata 112 a, and apply a NRZ modulation technique for direct modulationof the laser diode 108 a. Similarly, the TX processing unit 102 b mayreceive a stream of data 112 b, and apply a NRZ modulation technique fordirect modulation of the laser diode 108 b. The TX processing unit 102 cmay receive a stream of data 112 c, and apply a NRZ modulation techniquefor direct modulation of the laser diode 108 c. The TX processing unit102 d may receive a stream of data 112 d, and apply a NRZ modulationtechnique for direct modulation of the laser diode 108 d.

Example modulation techniques may include, but are not limited to, aquadrature amplitude modulation (QAM) technique, a phase-shift keying(PSK) technique, a frequency-shift keying (FSK) technique, anamplitude-shift keying (ASK) technique, non-return-to-zero (NRZ) linecoding, pulse-amplitude modulation (PAM), and any other suitablemodulation techniques.

A corresponding laser diode 108 may receive a corresponding signal andmay emit an optical carrier signal with a particular wavelengthaccording to the corresponding signal. The laser diode 108 may include avertical-cavity surface-emitting laser (VCSEL), a distributed feedback(DFB) laser, or another suitable laser.

In some embodiments, each of the laser diodes 108 in the laser array 107may be configured to emit an optical carrier signal with a differentwavelength. For example, the laser diode 108 a may emit an opticalcarrier signal with a wavelength λ₁. Similarly, the laser diode 108 bmay emit an optical carrier signal with a wavelength λ₂. The laser diode108 c may emit an optical carrier signal with a wavelength λ₃. The laserdiode 108 d may emit an optical carrier signal with a wavelength λ₄.Wavelengths λ₁, λ₂, λ₃, and λ₄ of the optical carrier signals may be ina wavelength range between 780 nm and 1000 nm or in another suitablewavelength range. Each two adjacent wavelengths λ₁, λ₂, λ₃, and λ₄ maybe spaced with a distance between about 15 nm and about 60 nm or anothersuitable spacing distance (e.g., 15 nm 15₂-λ₁≦60 nm, 15 nm su₃-λ₂≦60 nm,15 nm≦λ₄-λ₃≦60 nm).

The MUX 110 may multiplex the optical carrier signals that are outputfrom the laser array 107 onto the optical link 150 for transmission to areceiver (not shown in FIG. 1A). The MUX 110 may include a WDMmultiplexer.

FIG. 1B is a block diagram of an example structure of a multi-channelreceiver (receiver) 190, arranged in accordance with at least someembodiments described herein. The receiver 190 may include: a DEMUX 119;a photodiode array 121 that includes one or more photodiodes 120 a-120 d(also referred to individually or collectively as photodiode 120 orphotodiodes 120); one or more amplifiers 122 a-122 d (also referred toindividually or collectively as amplifier 122 or amplifiers 122); areceiver (RX) processing module 125 that includes one or more RXprocessing units 126 a-126 d (also referred to individually orcollectively as RX processing unit 126 or RX processing units 126); oneor more other suitable receiver components; or combinations thereof.

Although the receiver 190 is illustrated as a 4-channel receiver in FIG.1B, the receiver 190 may include more than 4 channels or fewer than 4channels. In some embodiments, the receiver 190 may additionally includetransmitting components that enable the receiver 190 to act as atransceiver for conducting bi-directional communications through one ormore optical links (e.g., the optical link 150).

In some embodiments, the DEMUX 119 may include a WDM demultiplexer. TheDEMUX 119 may receive a laser beam from the optical link 150. The laserbeam may include multiple optical carrier signals with differentwavelengths. The optical carriers may be corrupted by noise, loss, ISI,and/or distortion as the laser beam propagates through the optical link150.

The DEMUX 119 may split the laser beam into separate optical carriersignals according to the different wavelengths of the optical carriersignals, and may output the separate optical carrier signals to thephotodiodes 120 in the photodiode array 121. For example, the DEMUX 119may output an optical carrier signal with a wavelength λ₁ to thephotodiode 120 a, an optical carrier signal with a wavelength λ₂ to thephotodiode 120 b, an optical carrier signal with a wavelength λ₃ to thephotodiode 120 c, and an optical carrier signal with a wavelength λ₄ tothe photodiode 120 d.

The photodiodes 120 may convert the optical carrier signals receivedfrom the DEMUX 119 into analog signals. The amplifiers 122 may amplifythe analog signals received from the photodiodes 120. One or more of theamplifiers 122 may include a transimpedance amplifier (TIA) or otheramplifier circuitry. The RX processing units 126 may process thecorresponding signals to output streams of data 112.

Each of the RX processing units 126 may be associated with a channel andmay process signals transmitted through the channel. As used herein, achannel that is used to transmit an optical carrier signal with awavelength λ may be referred to as a channel with the wavelength λ or asa channel λ. The RX processing unit 126 a may process signalstransmitted through a channel with a wavelength λ₁. Similarly, the RXprocessing unit 126 b may process signals transmitted through a channelwith a wavelength λ₂. The RX processing unit 126 c may process signalstransmitted through a channel with a wavelength λ₃. The RX processingunit 126 d may process signals transmitted through a channel with awavelength λ₄. The channels λ₁-λ₄ may be different channels on theoptical link 150.

The RX processing unit 126 a may include a clock recovery unit 128 a, anequalizer 130 a, a coefficient estimation unit 132 a, and one or moreother suitable components. Similarly, the RX processing unit 126 b mayinclude a clock recovery unit 128 b, an equalizer 130 b, a coefficientestimation unit 132 b, and one or more other suitable components. The RXprocessing unit 126 c may include a clock recovery unit 128 c, anequalizer 130 c, a coefficient estimation unit 132 c, and one or moreother suitable components. The RX processing unit 126 d may include aclock recovery unit 128 d, an equalizer 130 d, a coefficient estimationunit 132 d, and one or more other suitable components.

The clock recovery units 128 a-128 d may be referred to individually orcollectively as clock recovery unit 128 or clock recovery units 128. Theequalizers 130 a-130 d may be referred to individually or collectivelyas equalizer 130 or equalizers 130. The coefficient estimation units 132a-132 d may be referred to individually or collectively as coefficientestimation unit 132 or coefficient estimation units 132. In someembodiments, the coefficient estimation unit 132 may be integrated intothe equalizer 130. In some embodiments, the clock recovery unit 128 mayinclude a phase-locked loop and any other circuitry components.

The clock recovery unit 128 of one of the RX processing units 126 may beconfigured to recover a clock signal associated with a particularchannel. For instance, the clock recovery unit 128 a may be configuredto recover the clock signal associated the channel λ₁. In someembodiments, the clock recovery unit 128 may recover the clock signalfor the particular channel using another clock signal recovered foranother channel as a reference clock signal. For example, clock recoveryunit 128 b may be configured to recover the clock signal for the channelλ₂ using the clock signal associated with the channel λ₁.

The clock recovery unit 128 may forward the clock signal, which isrecovered and/or based on a reference clock signal, to the equalizer 130of the RX processing unit 126. The equalizer 130 may use the clocksignal to perform equalization operations. The equalization operationsmay be based on a set of coefficients.

The coefficient estimation unit 132 of the RX processing unit 126 may beconfigured to estimate the set of coefficients for the equalizer 130from the RX processing unit 126. For example, the coefficient estimationunit 132 a may estimate the set of coefficients for the equalizer 130 afrom the RX processing unit 126 a.

In some embodiments, the coefficient estimation unit 132 may use anotherset of coefficients estimated for another equalizer 130 asinitialization coefficients to estimate the set of coefficients. Theinitialization coefficients may be used to initialize the set ofcoefficients at the beginning of a coefficient estimation process. Forexample, the coefficient estimation unit 132 b may use a set ofcoefficients estimated for the equalizer 130 b as initializationcoefficients. The initialization coefficients may be used to initializethe set of coefficients in the coefficient estimation unit 132 b.

The coefficient estimation unit 132 may configure the equalizer 130 withthe set of coefficients so that the equalizer 130 may equalize signalstransmitted through the particular channel. Equalization of the signalstransmitted through the particular channel may at least partiallycompensate ISI and/or other distortion caused by signal propagation.

Example equalization schemes applied in the equalizer 130 may include,but are not limited to, a feed-forward equalization (FFE) scheme, adecision feedback equalization (DFE) scheme, a decision directed leastmean square (DD-LMS) scheme, any other suitable equalization scheme,and/or some combination thereof.

With combined reference to FIGS. 1B and 2, an example process 200 ofperforming clock recovery and equalizer coefficient estimation in themulti-channel receiver 190 of FIG. 1B is described. The receiver 190 maybe a 4-channel receiver with a first channel λ₁ (e.g., λ₁=850 nm oranother wavelength), a second channel λ₂ (e.g., λ₂=880 nm or anotherwavelength), a third channel λ₃ (e.g., λ₃=910 nm or another wavelength),and a fourth channel λ₄ (e.g., λ₄=940 nm or another wavelength).

In some embodiments, the performance of the optical link 150 may beoptimized for the first channel λ₁ so that the first channel λ₁ may havea smaller ISI than the other channels λ₂, λ₃, and λ₄. For example, anOM3 link or an OM4 link may be optimized for data transmission throughthe first channel λ₁ with λ₄=850 nm, with a widest effective bandwidthat λ₁=850 nm when compared to other channels λ₂=880 nm, λ₃=910 nm, andλ₄=940 nm.

Initially, the clock recovery unit 128 a of the RX processing unit 126 aassociated with the first channel λ₁ may recover a first clock signalassociated with the first channel λ₁. The coefficient estimation unit132 a of the RX processing unit 126 a may estimate a first set ofcoefficients for the equalizer 130 a of the RX processing unit 126 ausing the recovered first clock signal. For example, when the clockrecovery unit 128 a is locked (e.g., an output from the clock recoveryunit 128 a is locked to the first clock signal), the coefficientestimation unit 132 a and the equalizer 130 a may cooperate with eachother to run a DD-LMS technique or another suitable technique to learnthe first set of coefficients optimized for the equalizer 130 a. Thefirst channel λ₁ may be associated with an eye diagram with an open eyeso that reliable clock recovery and equalization convergence may beachieved in the RX processing unit 126 a.

Next, the clock recovery unit 128 a may pass the first clock signal as areference clock signal 202 to the clock recovery unit 128 b of the RXprocessing unit 126 b associated with the second channel λ₂. In someembodiments, the second channel λ₂ may be a neighbor channel of thefirst channel λ₁. A first channel response of the first channel λ₁ maybe similar to or related to a second channel response of the secondchannel λ₂. For example, a channel transfer function of the firstchannel λ₁ may be similar to or related to a channel transfer functionof the second channel λ₂. As a result, the first clock signal and thefirst set of coefficients associated with the first channel λ₁ may beused to aid in recovery of a second clock signal and estimation of asecond set of coefficients associated with the second channel λ₂.

The clock recovery unit 128 b may use the reference clock signal 202from the clock recovery unit 128 a to recover the second clock signalassociated with the second channel λ₂. When the clock recovery unit 128b is locked (e.g., an output from the clock recovery unit 128 b islocked to the second clock signal), the coefficient estimation unit 132a may pass the first set of coefficients as initialization coefficients204 to the coefficient estimation unit 132 b of the RX processing unit126 b. The coefficient estimation unit 132 b may estimate the second setof coefficients for the equalizer 130 b of the RX processing unit 126 busing the recovered second clock signal and the initializationcoefficients 204. For example, the coefficient estimation unit 132 b mayinitialize the second set of coefficients using the initializationcoefficients 204 to aid in convergence of the estimation of the secondset of coefficients. The coefficient estimation unit 132 b and theequalizer 130 b may cooperate with each other to run a DD-LMS techniqueor another equalization technique to learn the second set ofcoefficients optimized for the equalizer 130 b.

Next, the clock recovery unit 128 b may pass the second clock signal asa reference clock signal 206 to the clock recovery unit 128 c of the RXprocessing unit 126 c associated with the third channel λ₃. In someembodiments, the third channel λ₃ may be a neighbor channel of thesecond channel λ₂. The second channel response of the second channel λ₂may be similar to or related to a third channel response of the thirdchannel λ₃. As a result, the second clock signal and the second set ofcoefficients associated with the second channel λ₂ may be used to aid inrecovery of a third clock signal and estimation of a third set ofcoefficients associated with the third channel λ₃.

The clock recovery unit 128 c may use the reference clock signal 206from the clock recovery unit 128 b to recover the third clock signalassociated with the third channel λ₃. When the clock recovery unit 128 cis locked, the coefficient estimation unit 132 b may pass the second setof coefficients as initialization coefficients 208 to the coefficientestimation unit 132 c of the RX processing unit 126 c. The coefficientestimation unit 132 c may estimate the third set of coefficients for theequalizer 130 c of the RX processing unit 126 c using the recoveredthird clock signal and the initialization coefficients 208. For example,the coefficient estimation unit 132 c may initialize the third set ofcoefficients with the initialization coefficients 208 to aid inconvergence of the estimation of the third set of coefficients. Thecoefficient estimation unit 132 c and the equalizer 130 c may cooperatewith each other to run a DD-LMS technique or another equalizationtechnique to learn the third set of coefficients optimized for theequalizer 130 c.

Next, the clock recovery unit 128 c may pass the third clock signal as areference clock signal 210 to the clock recovery unit 128 d of the RXprocessing unit 126 d associated with the fourth channel λ₄. In someembodiments, the fourth channel λ₄ may be a neighbor channel of thethird channel λ₃. The third channel response of the third channel λ₃ maybe similar to or related to a fourth channel response of the fourthchannel λ₄. As a result, the third clock signal and the third set ofcoefficients associated with the third channel λ₃ may be used to aid inrecovery of a fourth clock signal and estimation of a fourth set ofcoefficients associated with the fourth channel λ₄.

The clock recovery unit 128 d may use the reference clock signal 210from the clock recovery unit 128 c to recover the fourth clock signalassociated with the fourth channel λ₄. When the clock recovery unit 128d is locked, the coefficient estimation unit 132 c may pass the thirdset of coefficients as initialization coefficients 212 to thecoefficient estimation unit 132 d of the RX processing unit 126 d. Thecoefficient estimation unit 132 d may estimate the fourth set ofcoefficients for the equalizer 130 d of the RX processing unit 126 dusing the recovered fourth clock signal and the initializationcoefficients 212.

FIGS. 1B and 2 depict a 4-channel receiver 190 and 4 channels λ₁-λ₄. Insome embodiments, the process 200 may be applied in a multi-channelreceiver having more than four channels and associated with more thanfour WDM channels. Additionally, the process 200 may be applied in amulti-channel receiver having fewer than four channels and associatedwith fewer than four WDM channels.

FIG. 3 includes a graphic representation 300 that illustrates examplebandwidth-wavelength dependence on an example MMF link (e.g., an OM3 orOM4 link), arranged in accordance with at least some embodimentsdescribed herein. FIG. 3 illustrates a worst case of fiberbandwidth-wavelength dependence. For example, FIG. 3 shows a statisticalmodel for the wavelength dependence of the worst case OM4 fiberbandwidth. The solid curve shows only modal bandwidth, while the othertwo curves show the net bandwidth after including chromatic dispersion.

The bandwidth-wavelength dependence may indicate that an effectivebandwidth for a channel on the MMF link may depend on a wavelengthassociated with the channel. Four wavelengths (e.g., λ₁=850 nm, λ₂=880nm, λ₃=910 nm, and λ₄=940 nm) are illustrated with arrows 302, 304, 306,and 308, respectively. Compared to the wavelengths λ₂=880 nm, λ₃=910 nm,and λ₄=940 nm, the MMF link is optimized for best performance near thewavelength λ₁=850 nm. For example, the MMF fiber (e.g., an OM3 or OM4fiber) may have a widest fiber bandwidth near the wavelength λ₄=850 nmwhen compared to the wavelengths λ₂=880 nm, λ₃=910 nm, and λ₄=940 nm.

FIG. 4 includes graphic representations 400 that illustrate example eyediagrams 402, 404, 406, and 408 associated with different wavelengths onan MMF optical link, arranged in accordance with at least someembodiments described herein. Since an effective bandwidth on an MMFoptical link may depend on a wavelength of a channel as illustrated inFIG. 3, eye openings of eye diagrams may vary across different channelsassociated with different wavelengths. The eye diagrams 402, 404, 406,and 408 are simulated with a data rate of 25 Gb/s for a 300-meter OM4link, and may correspond to worst case fiber bandwidths for wavelengths850 nm, 880 nm, 910 nm, and 940 nm, respectively.

A channel with the wavelength 850 nm is associated with the eye diagram402, with a widest eye opening compared to the eye diagrams 404, 406,and 408. Reliable clock recovery and equalizer coefficient convergencemay be achieved with high confidence for the channel with the wavelength850 nm even if the channel has the worst case fiber bandwidth. A channelwith the wavelength 940 nm is associated with the eye diagram 408 withclosed eyes in the worst case. Signals transmitted through the channelwith the wavelength 940 nm may be severely degraded, and clock recoveryand equalization performed for the channel with the wavelength 940 nmmay not be reliable. However, the receiver may exploit similarity orcorrelation between neighbor channels (or adjacent channels) to providea reliable clock recovery and equalizer convergence for all the WDMchannels. For example, the receiver may perform a process identical orsimilar to the process 200 of FIG. 2 for reliable clock recovery andequalizer coefficient estimation for all the channels, beginning withthe channel (here with a wavelength of 850 nm) having the widest eyeopening and thus reliable clock and recovery equalizer coefficientconvergence.

FIG. 5 shows an example flow diagram of a method 500 of performing clockrecovery and equalizer coefficient estimation in a multi-channelreceiver, arranged in accordance with at least some embodimentsdescribed herein. The method 500 may be performed in whole or in part bya receiver (e.g., the receiver 190 of FIG. 1B). Although illustrated asdiscrete blocks, various blocks may be divided into additional blocks,combined into fewer blocks, or eliminated, depending on the desiredimplementation.

The method 500 may begin at block 502 in which a first clock signalassociated with a first channel may be recovered at a first clockrecovery unit of the receiver.

At block 504, first coefficients for a first equalizer associated withthe first channel may be estimated using the first clock signal.

At block 508, the first clock signal may be passed, as a reference clocksignal, to a second clock recovery unit associated with a secondchannel.

At block 510, the first coefficients may be passed as initializationcoefficients for a second equalizer associated with the second channel.

At block 512, a second clock signal associated with the second channelmay be recovered at the second clock recovery unit using the referenceclock signal.

At block 514, second coefficients for the second equalizer associatedwith the second channel may be estimated using the initializationcoefficients and the second clock signal recovered for the secondchannel.

In some embodiments, the estimation of the second set of coefficientsfor the second equalizer includes one or more additional operations. Forexample, in these and other embodiments, the estimation of the secondset of coefficients may include initializing the second set ofcoefficients using the first initialization coefficients. Additionallyor alternatively, the estimation of the second set of coefficients mayinclude estimating the second set of coefficients using an equalizationscheme after the initialization. The equalization scheme may include aFFE scheme, a DFE scheme, a DD-LMS scheme, another suitable equalizationscheme, or some combination thereof.

In some embodiments, the first channel has a first channel response thatis related to a second channel response of the second channel.Additionally or alternatively, the first channel may be associated witha wavelength for which an optical link, such as an MMF link, isoptimized. Accordingly, the first channel may include a smaller ISI thanthe second channel. Additionally, in some embodiments, the first channeland the second channel are associated with a first wavelength and asecond wavelength respectively that are within a range between about 800nm and about 1000 nm. In these and other embodiments, a channel spacingbetween the first channel and the second channel may be within a rangebetween about 15 nm and about 60 nm.

In some embodiments, the method 500 may continue performing operationssimilar to blocks 508, 510, 512, and 514 to recover clocks signals andto estimate coefficients for a third channel, a fourth channel, and/orany other numbers of channels. For example, the method 500 mayadditionally include passing the second clock signal as a referenceclock signal to a third clock recovery unit associated with a thirdchannel and recovering, at the third clock recovery unit, a third clocksignal associated with the third channel using the reference clocksignal. The method 500 may also include passing the second coefficientsas initialization coefficients for a third equalizer associated with thethird channel. The method 500 may also include estimating thirdcoefficients for the third equalizer using the second coefficients asthe initialization coefficients.

In these and other embodiments, the second channel may be a neighborchannel of the first channel, the third channel may be a neighborchannel of the second channel, and the fourth channel may be a neighborchannel of the third channel. Additionally or alternatively, the firstchannel may be associated with a wavelength of 850 nm, the secondchannel is associated with a wavelength of 880 nm, the third channel maybe associated with a wavelength of 910 nm, and the fourth channel isassociated with a wavelength of 940 nm.

In some embodiments, the first channel is associated with a wavelengthfor which an optical link has been optimized. The first channel mayaccordingly have a smaller ISI than the second channel. In addition, thesecond channel may have a smaller ISI than the third channel, and thethird channel may have a smaller ISI than the fourth channel.

One skilled in the art will appreciate that, for this and otherprocesses and methods disclosed herein, the functions performed in theprocesses and methods may be implemented in differing order.Furthermore, the outlined steps and operations are only provided asexamples, and some of the steps and operations may be optional, combinedinto fewer steps and operations, implemented with additional steps andoperations, or expanded into additional steps and operations withoutdetracting from the essence of the disclosed embodiments.

Some embodiments disclosed herein include an article of manufacture suchas a non-transitory computer storage medium having instructions storedthereon that are executable by a computing device to perform or controlperformance of operations included in the method 500 of FIG. 5, such asthe operations illustrated by blocks 502, 504, 508, 510, 512, and/or 514in FIG. 5, and/or variations thereof. The non-transitory computerstorage medium may be included in or may be accessible to a computingdevice such as the computing device 600 of FIG. 6 or a DSP unit thatincludes a processor and a memory. In some embodiments, thenon-transitory computer storage medium may be included in or may beaccessible to the receiver (e.g., the receiver 190 of FIG. 1B).

FIG. 6 is a block diagram that illustrates an example computing device600 that is arranged for implementing digital signal processingtechniques in a data communication system, arranged in accordance withat least some embodiments described herein. For example, for digitalsignal processing techniques may include clock recovery and equalizerestimation processes described herein. The computing device 600 or oneor more components thereof may be included in the receiver 190 of FIG.1B.

In a very basic configuration 602, the computing device 600 maytypically include one or more processors 604 and a system memory 606. Amemory bus 608 may be used for communicating between the processor 604and the system memory 606.

Depending on the desired configuration, the processor 604 may be of anytype including, but not limited to, a CPU, a μP, a μC, a DSP, or anycombination thereof. The processor 604 may include one or more levels ofcaching, such as a level one cache 610 and a level two cache 612, aprocessor core 614, and registers 616. The example processor core 614may include an arithmetic logic unit (ALU), a floating point unit (FPU),a digital signal processing core (DSP core), or any combination thereof.An example memory controller 618 may also be used with the processor604, or in some implementations the memory controller 618 may be aninternal part of the processor 604.

Depending on the desired configuration, the system memory 606 may be ofany type including, but not limited to, volatile memory (such as RAM),non-volatile memory (such as ROM, flash memory), or any combinationthereof. The system memory 606 may include an operating system (OS) 620,one or more applications 622, and program data 624. The application 622may include digital signal processing (DSP) algorithms 626, or otherapplication that may be arranged to perform one or more of the functionsas described herein including those described with respect to the method500 of FIG. 5. The program data 624 may include DSP data 628 that may bepulled into the application 622 for analysis. In some embodiments, theapplication 622 may be arranged to operate with the program data 624 onthe OS 620 such that implementations of a method for clock recovery andequalizer coefficient estimation such as the method 500 of FIG. 5, maybe provided as described herein.

The computing device 600 may have additional features or functionality,and additional interfaces to facilitate communications between the basicconfiguration 602 and any required devices and interfaces. For example,a bus/interface controller 630 may be used to facilitate communicationsbetween the basic configuration 602 and one or more data storage devices632 via a storage interface bus 634. The data storage devices 632 may beremovable storage devices 636, non-removable storage devices 638, or acombination thereof. Examples of removable storage and non-removablestorage devices include magnetic disk devices such as flexible diskdrives and hard-disk drives (HDD), optical disk drives such as compactdisk (CD) drives or digital versatile disk (DVD) drives, solid statedrives (SSD), and tape drives to name a few. Example computer storagemedia may include volatile and nonvolatile, removable and non-removablemedia implemented in any method or technology for storage ofinformation, such as computer-readable instructions, data structures,program modules, or other data.

The system memory 606, removable storage devices 636, and non-removablestorage devices 638 are examples of computer storage media. Computerstorage media includes, but is not limited to, RAM, ROM, EEPROM, flashmemory or other memory technology, CD-ROM, digital versatile disks (DVD)or other optical storage, magnetic cassettes, magnetic tape, magneticdisk storage or other magnetic storage devices, or any other mediumwhich may be used to store the desired information and which may beaccessed by the computing device 600. Any such computer storage mediamay be part of the computing device 600.

The computing device 600 may also include an interface bus 640 forfacilitating communication from various interface devices (e.g., outputdevices 642, peripheral interfaces 644, and communication devices 646)to the basic configuration 602 via the bus/interface controller 630.Example output devices 642 include a graphics processing unit 648 and anaudio processing unit 650, which may be configured to communicate tovarious external devices such as a display or speakers via one or moreA/V ports 652. Example peripheral interfaces 644 include a serialinterface controller 654 or a parallel interface controller 656, whichmay be configured to communicate with external devices such as inputdevices (e.g., keyboard, mouse, pen, voice input device, touch inputdevice) or other peripheral devices (e.g., printer, scanner) via one ormore I/O ports 658. The example communication device 646 may include anetwork controller 660, which may be arranged to facilitatecommunications with one or more other computing devices 662 over anetwork communication link via one or more communication ports 664.

The network communication link may be one example of a communicationmedia. Communication media may typically be embodied bycomputer-readable instructions, data structures, program modules, orother data in a modulated data signal, such as a carrier wave or othertransport mechanism, and may include any information delivery media. A“modulated data signal” may be a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia may include wired media such as a wired network or direct-wiredconnection, and wireless media such as acoustic, radio frequency (RF),microwave, infrared (IR), and other wireless media. The termcomputer-readable media as used herein may include both storage mediaand communication media.

The computing device 600 may be implemented as a portion of a small-formfactor portable (or mobile) electronic device such as a cell phone, apersonal data assistant (PDA), a personal media player device, awireless web-watch device, a personal headset device, anapplication-specific device, or a hybrid device that includes any of theabove functions. The computing device 600 may also be implemented as apersonal computer including both laptop computer and non-laptop computerconfigurations.

The present disclosure is not to be limited in terms of the particularembodiments described herein, which are intended as illustrations ofvarious aspects. Many modifications and variations can be made withoutdeparting from its spirit and scope, as will be apparent to thoseskilled in the art. Functionally equivalent methods and apparatuseswithin the scope of the disclosure, in addition to those enumeratedherein, will be apparent to those skilled in the art from the foregoingdescriptions. Such modifications and variations are intended to fallwithin the scope of the appended claims. The present disclosure is to belimited only by the terms of the appended claims, along with the fullscope of equivalents to which such claims are entitled. It is to beunderstood that the present disclosure is not limited to particularmethods, reagents, compounds, compositions, or biological systems, whichcan, of course, vary. It is also to be understood that the terminologyused herein is for the purpose of describing particular embodimentsonly, and is not intended to be limiting.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed is:
 1. A method of performing clock recovery andequalizer coefficient estimation in a multi-channel receiver, the methodcomprising: recovering, at a first clock recovery unit, a first clocksignal associated with a first channel; estimating a first set ofcoefficients for a first equalizer associated with the first channelusing the first clock signal; passing the first clock signal from thefirst clock recovery unit to a second clock recovery unit associatedwith a second channel; recovering, at the second clock recovery unit, asecond clock signal associated with the second channel using the firstclock signal as a reference clock signal to the second clock recoveryunit; passing the first set of coefficients as first initializationcoefficients to a second equalizer associated with the second channel;and estimating a second set of coefficients for the second equalizerusing the first initialization coefficients.
 2. The method of claim 1,wherein estimating the second set of coefficients for the secondequalizer comprises: initializing the second set of coefficients usingthe first initialization coefficients; and estimating the second set ofcoefficients using an equalization scheme after the initialization. 3.The method of claim 1, wherein the first channel includes a firstchannel response that is related to a second channel response of thesecond channel.
 4. The method of claim 1, wherein: the first channel isassociated with a wavelength for which an optical link is optimized; andthe first channel has a smaller inter-symbol interference (ISI) than thesecond channel.
 5. The method of claim 4, wherein the optical linkincludes a multi-mode fiber (MMF) link.
 6. The method of claim 1,further comprising: passing the second clock signal from the secondclock recovery unit to a third clock recovery unit associated with athird channel; recovering, at the third clock recovery unit, a thirdclock signal associated with the third channel using the second clocksignal as a reference clock signal to the third clock recovery unit;passing the second set of coefficients as second initializationcoefficients to a third equalizer associated with the third channel;estimating a third set of coefficients for the third equalizer based onthe second initialization coefficients; passing the third clock signalfrom the third clock recovery unit to a fourth clock recovery unitassociated with a fourth channel; recovering, at the fourth clockrecovery unit, a fourth clock signal associated with the fourth channelusing the third clock signal as a reference clock signal to the fourthclock recovery unit; passing the third set of coefficients as thirdinitialization coefficients to a fourth equalizer associated with thefourth channel; and estimating a fourth set of coefficients for thefourth equalizer based on the third initialization coefficients.
 7. Themethod of claim 6, wherein: the second channel is a neighbor channel ofthe first channel; the third channel is a neighbor channel of the secondchannel; and the fourth channel is a neighbor channel of the thirdchannel.
 8. The method of claim 7, wherein: the first channel isassociated with a wavelength of 850 nanometers (nm); the second channelis associated with a wavelength of 880 nm; the third channel isassociated with a wavelength of 910 nm; and the fourth channel isassociated with a wavelength of 940 nm.
 9. The method of claim 6,wherein: the first channel is associated with a wavelength optimized forperformance on an optical link and has a smaller inter-symbolinterference (ISI) than the second channel; the second channel has asmaller ISI than the third channel; and the third channel has a smallerISI than the fourth channel.
 10. The method of claim 1, wherein: thefirst channel is associated with a first wavelength within a rangebetween 800 nanometers (nm) and 1000 nm; the second channel isassociated with a second wavelength within the range between 800 nm and1000 nm; and a channel spacing between the first channel and the secondchannel is within a range between 15 nm and 60 nm.
 11. A multi-channelreceiver comprising: a first clock recovery unit configured to recover afirst clock signal associated with a first channel, the first clockrecovery unit configured to pass the first clock signal to a secondclock recovery unit associated with a second channel; a firstcoefficient estimation unit configured to estimate a first set ofcoefficients for a first equalizer associated with the first channelusing the first clock signal, the first coefficient estimation unitconfigured to pass the first set of coefficients to a second coefficientestimation unit; the second clock recovery unit configured to recover asecond clock signal associated with the second channel using the firstclock signal as a reference clock signal to the second clock recoveryunit; and the second coefficient estimation unit configured to estimatea second set of coefficients for a second equalizer associated with thesecond channel using the first set of coefficients as firstinitialization coefficients for the second equalizer.
 12. The receiverof claim 11, wherein the second coefficient estimation unit is furtherconfigured to: initialize the second set of coefficients using the firstinitialization coefficients; and estimate the second set of coefficientsusing an equalization scheme after the initialization.
 13. The receiverof claim 11, wherein the first channel has a first channel response thatis related to a second channel response of the second channel.
 14. Thereceiver of claim 11, wherein the first channel is associated with awavelength for which an optical link is optimized and includes a smallerinter-symbol interference (ISI) than the second channel.
 15. Thereceiver of claim 14, wherein the optical link includes one of an OM3fiber link and an OM4 fiber link.
 16. The receiver of claim 11, wherein:the second clock recovery unit is further configured to pass the secondclock signal from the second clock recovery unit to a third clockrecovery unit associated with a third channel; the second coefficientestimation unit is further configured to pass the second set ofcoefficients from the second coefficient estimation unit to a thirdcoefficient estimation unit as second initialization coefficients; andthe receiver further comprises: the third clock recovery unit configuredto: recover a third clock signal associated with the third channel usingthe second clock signal as a reference clock signal to the third clockrecovery unit; and pass the third clock signal from the third clockrecovery unit to a fourth clock recovery unit associated with a fourthchannel; the third coefficient estimation unit configured to: estimate athird set of coefficients for a third equalizer associated with thethird channel using the second initialization coefficients; and pass thethird set of coefficients from the third coefficient estimation unit toa fourth coefficient estimation unit as third initializationcoefficients; the fourth clock recovery unit configured to recover afourth clock signal associated with the fourth channel using the thirdclock signal as a reference clock signal to the fourth clock recoveryunit; and the fourth coefficient estimation unit configured to estimatea fourth set of coefficients for a fourth equalizer associated with thefourth channel using the third initialization coefficients.
 17. Thereceiver of claim 16, wherein: the second channel is a neighbor channelof the first channel; the third channel is a neighbor channel of thesecond channel; and the fourth channel is a neighbor channel of thethird channel.
 18. The receiver of claim 17, wherein: the first channelis associated with a wavelength of 850 nanometers (nm); the secondchannel is associated with a wavelength of 880 nm; the third channel isassociated with a wavelength of 910 nm; and the fourth channel isassociated with a wavelength of 940 nm.
 19. A multi-channel receivercomprising: a demultiplexer (DEMUX) that is configured to receive alaser beam from an optical link, wherein the optical link is optimizedfor a particular wavelength and the laser beam includes one or moreoptical carrier signals associated with other wavelengths; a first clockrecovery unit configured to receive a first optical carrier signal ofthe optical carrier signals from the DEMUX and to recover a first clocksignal from the first optical carrier signal; and a second clockrecovery unit configured to receive a second optical carrier signal ofthe optical carrier signals from the DEMUX, to receive a reference clocksignal from the first clock recover unit, and to recover a second clocksignal using the reference clock signal, wherein: the first opticalcarrier signal is associated with a first wavelength; the second opticalcarrier signal is associated with a second wavelength; the firstwavelength is between the second wavelength and the particularwavelength; and the reference clock signal is the first clock signal.20. The receiver of claim 19, further comprising a first coefficientestimation unit configured to: estimate a first set of coefficients fora first equalizer associated with the first channel using the firstclock signal; and pass the first set of coefficients as initializationcoefficients to a second equalizer associated with the second clockrecovery unit.